1995 Impala SS - Transmission Issues

[MagicRat]

I am not sure about the 4L60E, but back in the old days, it was easy to get a transmission to shift very firmly by changing the springs in the valve body. But a firm shifting transmission in not necessarily a stronger or better one.
Very firm shifts place excess strain on the gears, shafts and clutches of a transmission and, theoretically can lead to broken parts - parts which may live longer with softer shifts.

[LT1Silverhawk]

Originally Posted by MagicRat

I am not sure about the 4L60E, but back in the old days, it was easy to get a transmission to shift very firmly by changing the springs in the valve body. But a firm shifting transmission in not necessarily a stronger or better one.

Originally Posted by MagicRat

Very firm shifts place excess strain on the gears, shafts and clutches of a transmission and, theoretically can lead to broken parts - parts which may live longer with softer shifts.

From what I’ve been told, the 4L60E is basically a 700R4 that shifts electronically, so perhaps the idea of changing the springs to modify the shift’s firmness may still apply.

And I must add that the shifts from the Impala’s transmission were super hard. I noticed the shifts weren’t as bad after I installed a 180 degree thermostat (the car did not have one; just a housing). Perhaps this allowed the transmission fluid to get to a proper heat level?

When I brought up the firmness of the shifts to “The Shop’s” owner, he told me that is how the transmission was supposed to shift. I never did fully believe him.

But the transmission did go on me out of nowhere. No signs of trouble before this incident.

[j cAT]

Originally Posted by LT1Silverhawk

From what I’ve been told, the 4L60E is basically a 700R4 that shifts electronically, so perhaps the idea of changing the springs to modify the shift’s firmness may still apply.

And I must add that the shifts from the Impala’s transmission were super hard. I noticed the shifts weren’t as bad after I installed a 180 degree thermostat (the car did not have one; just a housing). Perhaps this allowed the transmission fluid to get to a proper heat level?

When I brought up the firmness of the shifts to “The Shop’s” owner, he told me that is how the transmission was supposed to shift. I never did fully believe him.

But the transmission did go on me out of nowhere. No signs of trouble before this incident.

my 96 impala the tranny oem shifts very smooth...
the only vehicle I ever owned that had the harsh 1-2 upshift is the 84 camaro with the 700r4 tranny..I still have this vehicle ...since 84..never did like that shift...as mentioned its not good..had it checked out at a shop also back in 86 said they all shift that way and I found this correct..

[LT1Silverhawk]

Originally Posted by j cAT

my 96 impala the tranny oem shifts very smooth...

Originally Posted by j cAT

the only vehicle I ever owned that had the harsh 1-2 upshift is the 84 camaro with the 700r4 tranny..I still have this vehicle ...since 84..never did like that shift...as mentioned its not good..had it checked out at a shop also back in 86 said they all shift that way and I found this correct..

Wow, a fellow Impala and an ’84 Camaro original owner! I’ve got an ’84 that is in project condition at the moment. Never worked on a carbureted car before so let’s see what I screw up. The last owner supposedly installed a 383 with a Powerglide with the intention of building a dragster. It cranks but it doesn’t fire up and I haven’t had a chance to find out why.

I would assume the stock transmission on Impalas do shift nice and smooth, which is another reason why their owners prefer them as their daily drivers. But the transmission in my car supposedly blew out once before when the 383 was installed.

Man, if I only had a way of showing you guys how crazy this car up shifted prior to going funky on me.

[j cAT]

with the 4l60e that you have ..when the transmision slips the PCM /computer detects this...then it commands maximum transmission line pressure to reduce the slipage..on engine shut down this will reset...then will again repeat if the transmission slips..

maximum pressure equals a harsh shift...I had this occur on my 2000 silverado one time...it was about 6 below zero F out ...

[LT1Silverhawk]

Originally Posted by j cAT

with the 4l60e that you have ..when the transmision slips the PCM /computer detects this...then it commands maximum transmission line pressure to reduce the slipage..on engine shut down this will reset...then will again repeat if the transmission slips..

Originally Posted by j cAT

maximum pressure equals a harsh shift...I had this occur on my 2000 silverado one time...it was about 6 below zero F out ...

I was told by “The Shop” owner that my transmission shifts off full-line pressure, which is why the transmission up shifted so hard.

So, if I understand this correctly, the PCM forces the transmission to shift off full-line pressure due a slippage it detects. Does this occur even with a reprogrammed PCM? Or I guess a better question is, would a reprogrammed PCM minimize the harshness of the up shift?

[j cAT]

Originally Posted by LT1Silverhawk

I was told by “The Shop” owner that my transmission shifts off full-line pressure, which is why the transmission up shifted so hard.

So, if I understand this correctly, the PCM forces the transmission to shift off full-line pressure due a slippage it detects. Does this occur even with a reprogrammed PCM? Or I guess a better question is, would a reprogrammed PCM minimize the harshness of the up shift?

yes it appears they programmed the pcm to max line pressure..this should not be set to this.. this can be changed with the proper equiptment..this stresses out the tranny as well as the u joints...not a comfortable ride..

with the use of mobil synthetic transmission fluid this transmission will normally shift very smooth and firm...the original dexron III was not up to this ...and this may be why this was programmed to the max line pressure to overcome the dexron III issues...

the synthetic fluid made a big improvement with my both 4l60 e trannys..

[LT1Silverhawk]

Originally Posted by j cAT

yes it appears they programmed the pcm to max line pressure..this should not be set to this.. this can be changed with the proper equiptment..this stresses out the tranny as well as the u joints...not a comfortable ride..

Originally Posted by j cAT

with the use of mobil synthetic transmission fluid this transmission will normally shift very smooth and firm...the original dexron III was not up to this ...and this may be why this was programmed to the max line pressure to overcome the dexron III issues...

the synthetic fluid made a big improvement with my both 4l60 e trannys..

That is some interesting and very important information that I was not aware off.

I’ve always had a feeling that there was an issue with the programming of the PCM because “The Shop” owner told me the previous owner was constantly sending off the PCM to be reprogrammed somewhere on the east coast. And I thought it may have something to do with the shifting but didn’t realize it was these severe.

Also, I noticed that at high speeds, there was a constant booming noise echoing throughout the car. It started usually after about hitting 75 mph. It just went ‘boom boom boom’. I wonder if it was related to this.

Although im not sure if the original fluid in the transmission is Dexron III, I guess I should’ve check and replaced the fluid. Now I know what do next time. Do you think Mobil 1 Synthetic (which I'm a huge fan of) will make a significant difference in shifts in a performance-built transmission?

[j cAT]

the boom noises I have no knowlege..I would post that after you get the transmission repaired to see if it is still there..

mobil synthetic is the tranny fluid I would use...with the tranny being repaired be sure they use the synthetic fluid...

dexron III is still available as I noticed it on the shelves of the auto part stores BUT there are no spec numbers on the bottle or where the fluid was made or who made it..............so this fluid should not be used in any thing..

have the PCM programed to restore the transmission operation to original..this may be difficult or easy if you can find a shop with the equiptment...I have never changed my pcm programing...years ago you could by a programer but not sure if those could change the tranny control...

[LT1Silverhawk]

Originally Posted by j cAT

the boom noises I have no knowlege..I would post that after you get the transmission repaired to see if it is still there..

Yeah its really weird and probably annoys the passengers. Driveshaft-related?

Originally Posted by j cAT

mobil synthetic is the tranny fluid I would use...with the tranny being repaired be sure they use the synthetic fluid...

Originally Posted by j cAT

dexron III is still available as I noticed it on the shelves of the auto part stores BUT there are no spec numbers on the bottle or where the fluid was made or who made it..............so this fluid should not be used in any thing..

Ok, Mobil synthetic is added to the shopping list.

How about rear end gear oil? Do you recommend the same?

Originally Posted by j cAT

have the PCM programed to restore the transmission operation to original..this may be difficult or easy if you can find a shop with the equiptment...I have never changed my pcm programing...years ago you could by a programer but not sure if those could change the tranny control...

I know there is one programmer out there called the LT1 Edit, but it’s $1500 for one PCM and the user has to be pretty advanced in the mechanics of the car to use it. I think there are a few reputable local ones. Otherwise, PCM For Less seems to come up a lot on message boards with good reviews.

Since the transmission shifts electronically, I would assume it can be reprogrammed through the PCM.



Other updates:

After having talked to all of you guys and having some sense knocked into me, I have abandoned the idea of dropping the transmission myself.

I will be contacting my coworker’s cousin this weekend to see what can be done about my car. If he seems well-versed, I’ll pass the job to him and help him out so I can learn along the way. Sounds like the best way to kill two birds with one stone.

Also, any recommendations on performance rebuild kits and torque converters (should they be needed)? There are so many options out there I don’t know which one to go with. if you know of a good guide, that would be very helpful.

Thank you guys...

[j cAT]

[quote=LT1Silverhawk;6089330]

How about rear end gear oil? Do you recommend the same?

the rear diff, if the stock limited diff takes, 80-90wt gl5 hypoy C .
since 1996 I have used the castrol brand of this gear oil with the 4 oz of rear diff additive required for the clutches,,,,when doing this clean the rear diff out and use the proper diff cover gasket [DO NOT BLOCK THE DRAIN BACK HOLES]...these holes return oil from the rear wheel bearings.

[LT1Silverhawk]

Originally Posted by j cAT

the rear diff, if the stock limited diff takes, 80-90wt gl5 hypoy C .
since 1996 I have used the castrol brand of this gear oil with the 4 oz of rear diff additive required for the clutches,,,,when doing this clean the rear diff out and use the proper diff cover gasket [DO NOT BLOCK THE DRAIN BACK HOLES]...these holes return oil from the rear wheel bearings.

The rearend was rebuilt with 3.90 gears. Im not sure what kind of oil is in there already.

Perhaps I'll give "The Shop" a call to see if they remember...

[j cAT]

Originally Posted by LT1Silverhawk

The rearend was rebuilt with 3.90 gears. Im not sure what kind of oil is in there already.

Perhaps I'll give "The Shop" a call to see if they remember...

I forgot that this has been changed out to the 3.90...do a search on this in the impalass forums as most likely this has been done by someone there.

[LT1Silverhawk]

Hi All,


So, during the time between my last post and today, I’ve doing as much research as possible and found the following:



a. Local Performance Transmission Builders – nothing so far except a local performance auto parts store (Team C Performance) and coworker’s cousin who is a certified mechanic at a BMW dealership and works on GM cars for a hobby. He is estimating $400 in labor. Verdict: Best route if I want to learn how to rebuild a transmission and save some moola.


b. Rebuild Kits – found a few from several companies on Jegs and Summit but not sure which one to go with yet.


c. Harsh Shifting – can be eliminated to some extent by reprogramming the PCM.


d. Transmission Replacement – A guy in San Diego area has been advertising his expertise in LT1s and Impalas (over ten years) and works on these cars and builds performance-oriented LT1s. I gave him a call and he says he’s got a performance transmission with a TCI converter that was used for about 200 miles (the car was hit) for $1800. He also has a stock 4L60E transmission with 55,000 miles for $500. He says as long the car is not pushed, the stock transmission should hold up for a while until I am ready for a performance-oriented transmission. Verdict: The stock replacement is the cheapest route to take until I have enough money to put in the right transmission the first time (the second time around, of course). The car will keep moving in the mean time.


e. T-56 Swap – Someone created a booklet and posted it on the Impala SS forum that offers a complete walk through of the entire installation process. Can be done in a weekend with help. Verdict: Perhaps the most expensive and challenging option. But super cool and kind of rare.


f. 3.90: Gear Oil – haven’t found any information on that yet.


g. “Boom Boom” Noise – Possible vibrations from the driveshaft at high rpms.


Still haven’t heard anything from the shop where the car presently sits. I’ve been too busy to call. I’ll stop by on Friday to see what’s up but I’m not expecting any answers.

[j cAT]

great info keep us updated.

the rear diff could be a locker type diff. see if you can pull numbers off the gears..